This invention refers to a cooling device, a method for cooling an object as well as to a cooling system which is appropriate in particular for switch cupboards.
By heat we understand an energy which exists in form of irregular atomic or molecular movements. Since on the basis of the second main thermodynamics theorem, the entropy (disorder) of a system can only increase or remain the same, for all practical transformations of an energy type into another energy type, a generation of heat does take place, whereby this heat energy is lost for the transformed useful energy. Thus, the effort is made to maintain this xe2x80x9cwaste heatxe2x80x9d as low as possible or to avoid that it does develop at all. However, because of the physical or thermodynamic regularities, such efforts to avoid waste heat are subject to theoretical limits which cannot be exceeded. Therefore, it is unavoidable that for many practical processes waste heat develops which is to be dissipated as efficiently as possible in order to guarantee the operation of the corresponding system.
An important example for waste heat generating processes consists in electrical processes as they appear in electrotechnical installations, switchboards, electric circuits or microelectronic components. Because of friction processes of the electrons which carry the electric line, a power loss which is proportional to the resistance R and to the second power of the power loss developing as waste heat. The waste heat results, as explained in the introduction, in an increase of the kinetic energy of the atoms and molecules or in an increase of their oscillation amplitude in the solid-state grid.
For cooling objects, various methods are known which are fundamentally based on the principle that a cooler object (heat sink) is connected with a warmer object (source of heat) in such a way that, by reason of the thermodynamic tendency to take on a state of equilibrium, heat energy flows over from the source of heat to the heat sink. The transfer of heat energy can take place on the one hand by conduction of heat by which, at a microscopic level, the movement energy of the atoms or of the molecules is transferred by shock processes from one particle to the other. The rate with which heat energy is transferred by conduction of heat is proportional to the contact surface participating in the thermal conductibility xcex as well as to the temperature gradient (temperature difference per interval) between the source of heat and the heat sink.
A further mechanism for the transport of waste heat is the convection. Here, the waste heat is first transferred to a movable medium such as for example a cooling liquid and then carried-off with the matter of this cooling medium. The speed with which this carrying-off takes place is determined by the rate of motion of the cooling medium and can thus reach relatively high values.
Finally, a further mechanism for the dissipation of waste heat consists in the reradiation of heat energy in form of electromagnetic heat radiation. The transport speed of the heat energy is the light velocity, whereby the rate of the heat dissipation according to the Stefan Bolzmann law is proportional to the reradiating surface as well as to the fourth power of the temperature difference. This means that in particular for high temperature differences between the source of heat and the heat sink a high rate of heat dissipation can be achieved through reradiation.
For the carrying-off of waste heat as it develops, for example, for electronic components in switch cupboards, the matter is to carry off the waste heat quickly and safely away from the place of origin. Otherwise, it can come namely to a overheating and thus to a destruction of electronic components. Thus, for example, ventilation devices are used in which a strong fan blows cooling air along the surface of the components and thus ensues a quick carrying-off of heat by convection. Furthermore, it is known to apply large surface cooling bodies made of metal on components to be cooled such as, for example, microprocessors, whereby these cooling bodies should assure a quick transmission of the loss heat from the place of origin and the transfer thereof to the convection cooling. For these known solutions, it is disadvantageous that they require considerable constructional efforts with mechanically moved components which are highly subject to wear. Furthermore, the efficiency of such devices often leaves much to be desired and the cooling systems are connected with the use of substances which are harmful to the environment.
Aim, Solution, Advantage
The aim of this invention was to make available a novel cooling device which should be cheap, long wearing, as maintenance-free and simple as possible to construct without being harmful to the environment. Furthermore, the cooling device should guarantee a high efficiency for a cooling capacity also at low temperatures, preferably to xe2x88x9280xc2x0 C., and should be adapted to different technical general conditions. Moreover, it should be possible to operate the cooling device as autonomously as possible with batteries or accumulators.
The cooling device according to the invention contains accordingly an electron emission layer which is to be applied to the object to be cooled, furthermore a suction electrode placed at a distance from the electron emission layer as well as a source of voltage, the negative pole of which is connected with the electron emission layer and its positive pole with the suction electrode.
The cooling device transports the waste heat to be carried-off with the aid of electrons. Normally, electrons are bound inside a solid by chemical bonds for certain atoms or molecules. However, for many solids such as in particular metals, by reason of quantum-mechanical level superpositions, there develop (energetic) conduction bands in which the valency electrons can move freely inside the solid. Such valency electrons also transport heat energy with their energy of movement and moreover are the carriers of the electric current conduction when an electric source of voltage is connected with the solid. The space of motion of the valency electrons is substantially limited to the inside of the solid. In the solid, there are electrons in a so-called potential well which they can only leave when they carry along a correspondingly high energy of movement which allows them to overcome the potential stage at the surface of the solid. The energy necessary for the electrons of the upper valence band of a solid for leaving the solid is designated as activation energy and constitutes a matter constant. Since, by reason of the statistical distribution of heat energy in a solid, a few electrons always have a very high energy which lies over the activation energy, a few electrons always can leave the solid. This means that such a solid is surrounded by an xe2x80x9celectron cloudxe2x80x9d in direct vicinity of the surface. The quantitative description of the flow of the released electrons ensues by the so-called Richardson effect equation. For temperatures around 20xc2x0 C., the released electrons typically cover distances with a velocity of approximately 3500 m/s.
This invention now uses this effect of the electrons which are released for the dissipation of waste heat. In order to achieve this aim, the electron emission layer is provided which is to be connected with the object to be cooled. Eventually, the electron emission layer can also be configured as a part of the object to be cooled itself. The electron emission layer first absorbsxe2x80x94for example over heat conductionxe2x80x94waste heat from the object to be cooled. This heat energy is stored among others in the valency electrons of the electron emission layer. Thus, the fraction of the high energy electrons which overcome the activation energy and which thus can leave the electron emission layer rises. The discharged electrons carry along the inherent kinetic energy from the electron emission layer. In other words, they draw off energy from this layer. Without further measures, it would hower quickly come to a state of equilibrium between the electron cloud and the electron emission layer, i.e. that per unit of time exactly as many electrons would come out the electron emission layer than electrons from the electron cloud would come back into the electron emission layer. Thus, a net dissipation of heat energy could not take place. In order to interrupt this equilibrium and in order to assure a continuous carrying-off of heat energy from the electron emission layer, according to the invention the suction electrode is provided for at a distance of the electron emission layer. By feeding a source of voltage between the suction electrode (anode) and the electron emission layer (cathode), a potential drop is generated between the two electrodes which draws the discharged electrons to the suction electrode. The electrons are taken over then into the solid matter of the suction electrode and led to the source of voltage. The electrons are thus continuously removed from the electron cloud around the electron emission layer so that a discharge of high-energy electrons from the electron emission layer effectively takes place. As already explained above, the electrons move then with a very high velocity of typically 3500 m/s so that an extremely quick heat transport takes place from the electron emission layer, which is connected as a cathode according to the polarity of the source of voltage, to the suction electrode which is connected as an anode.
The advantages of the cooling device according to the invention are to be found in an efficient and quick transport of waste heat. This transport takes place without expensive constructional measures and without the use of movable mechanical parts so that also practically no wear can take place and the device can be operated maintenance-free for a long time, typically ten years. A further advantage of the cooling according to the invention is that it works absolutely silently and practically does not require any additional space for cooling elements.
In a further development of the invention, a grid is placed between the electron emission layer and the suction electrode, this grid being connected with a preferably adjustable source of voltage. The potential course can be varied by such a grid on the way between the electron emission electrode and the suction electrode and can be adjusted at will so that the flow of the high-energy electrons from the electron emission layer to the suction electrode can be regulated. To this aim, a voltage negative with respect to the electron emission layer is applied to the grid, this voltage acting repulsively onto the electrons of the electron cloud around the electron emission layer. The grid thus controls the potential course on the way between the electron emission layer and the suction electrode. The electrode current is controlled with the aid of the Coulomb""s forces in order to prevent a water formation inside the cooling space. Moreover, it is thus possible to configure variably the cooling capacity according to the technical requirements. Instead of a grid, a multiple grid can also be used in order to improve the control. Thus, the grid prevents the passage of the positive voltage of the suction electrode (anode) to an extent which can be adjusted by the negative voltage at the grid. An electric power draw of the grid itself does not take place. In this way, the cooling capacity of the cooling device can be exactly adapted to the respective needs. In particular, it can be avoided that at the beginning of a cooling process a too high cooling capacity develops which could result in an icing of the object to be cooled, or of the electron emission layer.
According to a further configuration according to the invention, the surface of the electron emission layer can contain an alkaline earth metal, preferably cesium (Cs) or barium (Ba) or electrides or mixtures thereof or be entirely made of these substances. For alkaline earth metals, the activation energy is comparatively low so that already a few electron volt (eV) are sufficient to release electrons for a heat transport. Electrides are special salts which have a very loose bond of the valency electrons, what again contributes to an easy availability of the heat transporting electrons.
Furthermore, said substances can be placed preferably on a layer which contains tungsten (W) or which is made thereof. A combination of alkaline earth metals with tungsten or with comparable elements results in a further reduction of the activation energy and thus in an easier release of the electrons necessary for the heat transport.
According to a further improvement of the invention, the surface of the electron emission layer has a surface profile shaping, for example a three-dimensional structure, which is created by forming recesses (ridges, grooves, channels). However, any other conformation of the surface structure is also conceivable when it achieves the purpose to increase the size of the contact surface between the electron emission layer and the environment so that the rate of the discharged electrons increases correspondingly. In the same way as the electron emission layer is provided with a surface profile shaping, the surface of the suction electrode can be provided with a profile shaping on its side turned to the electron emission layer.
Preferably, a vacuum is produced between the electron emission layer and the suction electrode. This vacuum assures that the free distance covered by the electrons is sufficiently big so that they can fly without collisions with gas atoms or gas molecules from the electron emission layer to the suction electrode. Such a vacuum thus considerably contributes to the increase of the heat transmission velocity and efficiency.
According to a further development of the cooling device according to the invention, this cooling device contains a device for generating a magnetic field in the area of the surface of the electron emission layer. Such a magnetic field acts positively and with a directing effect onto the movement of the electrons which induce the heat transfer. Because of the Lorentz force, the electrons move in a magnetic field on circuits or loops around the magnetic field lines. For a magnetic field directed preferably vertically to the surface of the electron emission layer, this magnetic field thus results in that movement components of the electrons directed tangentially to the surface are deviated to circular orbits. On the other hand, movements of the electrons vertically to the surface of the electron emission layer (i.e. in direction of the magnetic field) are not influenced by the magnetic field. In this manner, the electrons discharged from the electron emission layer are held quasi captured over the surface, whereby however their movement vertically to the surface and thus to the suction electrode is not impaired. Furthermore, with the aid of the magnetic fields, the discharge energies of the elements such as, for example, barium can be reduced by a few electron volt so that discharge energies of less than 1,0 eV can be obtained.
According to a further development of the invention, the electron emission layer contains radioactive elements which undergo a xcex2-decay (xcex2-emitter). Electrons are spontaneously released by the xcex2-decay so that electrons are still available in the electron emission layer even, due to low temperatures, there are not enough thermally activated electrons. A safety cooling can thus also be realized in case of lack of beam potential at the suction electrode.
The electron emission layer can also be configured as a thin-layer film on a highly conductive material, preferably on gold. Such an electrically and thermally conductive material assures a good transport of the waste heat inside the electron emission layer.
Finally, in a further development of the suction electrode, the suction electrode can show projecting parts such as, for example, edges which make possible a discharge of the electrons from the hot electron emission layer and make available the shortest way from the cathode to the anode.
Furthermore, the invention relates to a method for cooling an object which is characterized in that an electron emission layer is placed on the object and a suction electrode at a distance to this electrode emission layer and that the electron emission layer is applied opposite to the suction electrode to a negative electric potential. This means that the electron emission layer is operated as a cathode and the suction electrode as an anode. Due to the method according to the invention, a cooling can take place in the way explained above in the context of the cooling device, cooling by which electrons transport thermal energy from the electron emission layer to the suction electrode. The surface properties become positively apparent on the electron emission layer. Since namely the xe2x80x9chotxe2x80x9d electrons remain particularly often at the surface and the bonding force at the surface is reduced compared to the inside of the solid, it is very likely that these electrons leave the surface and move to the suction electrode. When leaving the solid, the electrons carry along their energy of movement as individual thermal energy. This causes a temperature reduction of the electron emission layer. The energy transported by the electrons arrives to the anode which slowly heats up. Therefore, it is generally necessary to cool the anode (suction electrode) with other methods which can be more conventional. However, it is important there that a quick and efficient carrying-off of heat takes place from the critical components, such as for example microprocessors or electronic circuits, so that the heat can then be further distributed and carried-off from the suction electrode under less critical conditions. The suction electrode can be cooled in particular by a separate Peltier effect or a thermoacoustical effect.
The flow of the electrons from the electron emission layer to the suction electrode is controlled by an electric potential which is applied to a grid placed between the electron emission layer and the suction electrode. Due to such an electric potential, the intensity of the electron flow from the electron emission layer to the suction electrode can be controlled powerless. A further degree of freedom is thus obtained which can be used for a manual or automatic control depending on the situation.
Furthermore, the invention relates to a cooling system which contains a secondary cooling which can be realized in particular in form of a convection cooling (for example by air or water). The cooling system is characterized in that it contains a cooling device of the above explained type to be applied on the object to be cooled, the suction electrode of which is cooled by the secondary cooling. Such a cooling system can be used for example in switch cupboards of electric components in which, due to electric currents, there raises a high volume of waste heat which is to be carried-off from the place of origin as quickly as possible for protecting the components. However, the cooling system can also be advantageously used for many other heat producers such as, for example, for light generators. dr
The invention will be described below with an example with reference to the annexed drawings.
FIG. 1 shows a schematic representation of the function principle of the invention.
FIG. 2 shows a cooling system according to the invention.
FIG. 3 shows a special layer structure of the cooling system.
FIG. 4 shows the cooling flow of a cooling device with Ba on W depending on the loss temperature.
FIG. 5 shows the cooling current of a cooling device with Cs on W depending on the loss temperature.
FIG. 6 shows the radiation energy of electron flows on the cathode side depending on the temperature difference.
FIG. 7 shows a schematic representation of the surface of an electron emission layer with a surface profile shaping.
FIG. 8 shows a side view of a suction electrode and of the electron emission layer with facing surface profiles.